Why can't I pose my armature after setting a keyframe?
Make sure you are editing the bone and adding the keyframes in Pose Mode. 


Go to the starting frame.  
Make the Location/Rotation change you want on the bone 
Press the i key and select Loc/Rot


This establishes your starting position and rotation.


Go to the end frame.
Make the Location/Rotation change on the bone.
Press the i key and select Loc/Rot.


This establishes your end position and rotation

Your animation should play as expected.